The purpose of these studies is to generate and characterize novel variants of murine mammary tumor viruses (MMTVs) and to employ these variants toward the development of in-vitro co-carcinogenesis transformation assays of murine and human mammary epithelial cells. Major accomplishments in this area during the last year include: (1) the selection of MMTV host range variants that have the ability to efficiently infect cells productively in-vitro; (2) development of cell substrates for mammary cell infectivity and transformation assays: (a) refined culture techniques have been estabished for the propagation of primary and secondary cultures of murine, rat, and human mammary epithelial cells and (b) cloning and characterization of cells have been obtained from cell lines derived from normal murine mammary glands; (3) MMTV host range variants have been utilized in combination with chemical carcinogens and tumor promotors, to develop in-vitro transformation assays for mammary epithelial cells.